Reorder buttons in Drawing Selection Pane
 

I have been using Excel do create some drawings and found the Excel drawing
tools quite powerful. One feature I use is to enable the Selection Pane,
and then move individual shapes more to the front or to the back. After
using this for a while, the Reorder buttons have stopped working. What
could cause that? How can I get them back? Is there a limit to how many
individual shapes are allowing in one worksheet or workbook?